The use of magnesium in structural applications is growing rapidly. Magnesium is generally alloyed with any of aluminum, manganese, thorium, lithium, tin, zirconium, zinc, rare earth metals or other alloys to increase its structural stability. Such magnesium alloys are often used where a high strength to weight ratio is required. The appropriate magnesium alloy can also offer the highest strength to weight ratio of the ultra light metals at elevated temperatures. Further, alloys with rare earth or thorium can retain significant strength up to temperatures of 315.degree. C. and higher. Structural magnesium alloys may be assembled in many of the conventional manners including riveting and bolting, arc and electric resistance welding, braising, soldering and adhesive bonding. The magnesium-containing articles have uses in the aircraft and aerospace industries, military equipment, electronics, automotive bodies and parts, hand tools and in materials handling. While magnesium and its alloys exhibit good stability in the presence of a number of chemical substances, there is a need to further protect the metal, especially in acidic environments and in salt water conditions. Therefore, especially in marine applications, it is necessary to provide a coating to protect the metal from corrosion.
There are many different types of coatings for magnesium which have been developed and used. The most common coatings are chemical treatments or conversion coatings which are used as a paint base and provide some corrosion protection. Both chemical and electrochemical methods are used for the conversion of magnesium surfaces. Chromate films are the most commonly used surface treatment for magnesium alloys. These films of hydrated, gel-like structures of polychromates provide a surface which is a good paint base but which provides limited corrosion protection.
Anodization of magnesium alloys is an alternative electrochemical approach to provide a protective coating. At least two low voltage anodic processes, Dow 17 and HAE, have been commercially employed. However, the corrosion protection provided by these treatments remains limited. The Dow 17 process utilizes potassium dichromate, a chromium (VI) compound, which is acutely toxic and strictly regulated. Although the key ingredient in the HAE anodic coating is potassium permanganate, it is necessary to use a chromate sealant with this coating in order to obtain acceptable corrosion resistance. Thus in either case, chromium (VI) is necessary in the overall process in order to achieve a desirable corrosion resistant coating. This use of chromium (VI) means that waste disposal from these processes is a significant problem.
More recently, metallic and ceramic-like coatings have been developed. These coatings may be formed by electroless or electrochemical processes. The electroless deposition of nickel on magnesium and magnesium alloys using chemical reducing agents in coating formulation is well known in the art. However, this process also results in the creation of large quantities of hazardous heavy metal contaminated waste water which must be treated before it can be discharged. Electrochemical coating processes can be used to produce both metallic and nonmetallic coatings. The metallic coating processes again suffer from the creation of heavy metal contaminated waste water.
Non-metallic coating processes have been developed, in part, to overcome problems involving the heavy metal contamination of waste water. Kozak, U.S. Pat. No. 4,184,926, discloses a two-step process for forming an anti-corrosive coating on magnesium and its alloys. The first step is an acidic chemical pickling or treatment of the magnesium work piece using hydrofluoric acid at about room temperature to form a fluoro-magnesium layer on the metal surface. The second step involves the electrochemical coating of the work piece in a solution comprising an alkali metal silicate and an alkali metal hydroxide. A voltage potential from about 150-300 volts is applied across the electrodes, and a current density of about 50-200 mA/cm.sup.2 is maintained in the bath. The first step of this process is a straight forward acid pickling step, while the second step proceeds in an electrochemical bath which contains no source of fluoride. Tests of this process indicate that there is a need for increased corrosion resistance and coating integrity.
Kozak, U.S. Pat. No. 4,620,904, discloses a one-step method of coating articles of magnesium using an electrolytic bath comprising an alkali metal silicate, an alkali metal hydroxide and a fluoride. The bath is maintained at a temperature of about 5.degree.-70.degree. C. and a pH of about 12-14. The electrochemical coating is carried out under a voltage potential from about 150-400 volts. Tests of this process also indicates that there remains a need for increased corrosion resistance.
Based on the teachings of the prior art, a process for the coating of magnesium-containing articles is needed which results in a uniform coating with increased corrosion resistance. Further, a more economical coating process is needed which has reduced apparatus demands and which does not result in the production of heavy metal contaminated waste water.